1. Field of the Invention
This invention relates to the field of automatic transmissions for automotive vehicles and more particularly pertains to belt driven automatic transmissions that produce continually variable drive ratios.
2. Description of the Prior Art
Automatic transmissions having an input pulley driven from the engine shaft and an output pulley connected to the vehicle wheel have been developed to produce a continually variable ratio of the speed of the output pulley in relation to the speed of the input pulley by varying the radius at which an endless drive belt or chain engages the respective pulleys. Steel chains that transmit torque between the pulleY assembly either by tension or compression developed in the chain must operate in a lubricated environment in order to realize an acceptable service life. However, v-belts made of rubber and composite materials can be substituted for the steel chain to engage conical surfaces formed on the input pulley and output pulley. The coefficient of friction on the adjacent surface of the v-belt and the pulley sheaves is approximately five times higher than the coefficient of friction on the lubricated steel belts and pulley surfaces. The higher coefficient of friction allows the clamping loads applied to the displaceable sheaves of the pulley assembly to be substantially reduced in comparison to the clamping loads required to engage lubricated steel belts on these surfaces. Lower clamping load reduces the cost of the transmission and allows substantially lower hydraulic actuating pressures to transmit torque between the pulleys.
However, in order to realize these advantages, the surfaces of the pulleys that engage the v-belt, the belt surfaces and the space within which the belt and pulleys operate should be entirely free from lubricant, clean and dry.
Automatic transmissions of this type include a source of high pressure hydraulic fluid connected to hydraulic cylinders whose pressure moves a piston, and a displaceable sheave connected to the piston to vary the effective radius at which the belt engages the pulley surfaces. The hydraulic actuation system and the lubrication system of the transmission must be sealed effectively to prevent the egress of hydraulic fluid into the space in which the sheaves and pulley operate. To maintain a dry operating environment for the belt requires the use of dynamic pressurized oil seals.
One sheave of each pulley assembly is axially displaceable and therefore requires support that permits rotation and axial displacement in a non-lubricated environment. The tensile force developed in the belt that transmits torque between the pulley assemblies produces a large bending moment at the hub of the fixed driven sheave. Support for the sheaves on the shafts on which they are supported requires that the moment tending to rotate the sheave on the shaft be reacted by a couple having a substantial couple arm. This requirement and the requirement for hydraulic sealing of the sheaves has been an important obstacle in the development of transmissions of this type that use a rubber belt to transmit power between the pulleys.